feat: add web support

This PR formats all code with dart format and adds a step to the CI so that it will be checked on every push and PR.

.github/workflows/flutter-format.yml

@@ -0,0 +1,20 @@
+name: Flutter Check Format
+
+on:  
+  pull_request: {}
+  push: {}
+
+jobs:
+  release:
+    name: Build
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v1
+
+      - name: Setup Flutter
+        uses: subosito/flutter-action@v1
+        with:
+          channel: stable
+      - name: Check Dart Format
+        run: dart format --set-exit-if-changed .

android/app/build.gradle

@@ -41,7 +41,7 @@ android {
 
     defaultConfig {
         applicationId "io.vikunja.app"
-        minSdkVersion 33
+        minSdkVersion 21
         targetSdkVersion 34
         testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
         versionCode flutterVersionCode.toInteger()